156 Opinions ofArijtotle 3 Roger Bacott, &c. [Book III. 



To this may be added the opinion of Ariftotle, who 

 fuppofed that rainbows, haios, and mock funs, were 



occafioned 



culty mould occur, he will probably find it removed by referring 

 to this note. 



1. Lip, lit is a matter, the particles of which are extremely fmaJI, 

 wlii-ii, by ftriking en our vifual organs, gives us the fenfation of 

 feeing. 



2. The particles of light are emitted from what are called lumi- 

 nous bodies, fuch as the fun, a fixe, a torch, or candle, &c. &c. ; 

 it is reflected or fent back by what are termed opake bodies, or 

 thofe which have no power of affording light in themfelves. 



3. Light, whether emitted or reflected, always moves \v\ftrait or 

 JireB lines, as may eafily be proved by looking into a bent tube 

 which evidently obftruds the progrefs of the light in direft lines. 



4. By a ray of ligiit is ufua'ly meant the leail particle of light 

 that can be either intercepted or fcparated from the reft. A 

 learn of light is generally ufed to expreis fomething of an aggre- 

 gate or mafs of fight greater than a fingle ray. 



5. Parallel rays are fuch as proceed equally diftant from each 

 other through their whole courfe. The diftance of the fun from 

 the earth is fo immenfe, that rays proceeding from the body of 

 that luminary are generally regarded as parallel. 



6. Converging rajs are fuch as, proceeding from any body, ap- 

 proach nearer and nearer to each other, and tend to unite in a 

 point. The form of rays thus tending to a union in a fingle 

 point has been compared to that of a candle extinguimer ; it is in 

 faft a per fed cone. 



7. Diverging rays are thofe which, proceeding from a point, 

 continue to recede from each other, and exhibit the form of an in- 

 verted cone. 



.9. A fmall objeft, or a fma.H fingle point of an objeft, from 

 which rays of light diverge or indeed proceed in any direction, is 

 fometimes called the radiant, or radiant point. 



o. Any parcel of rays, diverging from a point, confidered as fe- 

 parate from the reft, is called a pencil of rays. 



10. The foe us of rajs is that point to which converging rays 

 tend, and in which they unite and inte,rfel or crofs each other. 

 It may be confidered as the apex or point of the cone ; and it is 

 called the focus (or fire place) becaufe it is the point at which 

 burning- glaffes burn mod intenfely. 



11. The virtual or imaginary focus is that fuppofed point be- 

 hind a mirror or looking-glafs, where the rays would have natu- 

 rally united, had they not been intercepted by the mirror. 



